Tandem organic electroluminescent device

ABSTRACT

A tandem organic electroluminescent device. The tandem organic electroluminescent device comprises a substrate having a pixel thin film transistor. A rib with chambered corners is formed on the substrate, surrounding a display region. A protrusion is formed in the display region. A plurality of organic light emitting diodes is stacked vertically in the display region, covering the protrusion, wherein each organic light emitting diode comprises a top electrode, an organic electroluminescent layer, and a bottom electrode. The bottom electrode of the bottommost organic light emitting diode is electrically connected to the pixel thin film transistor. A common electrode electrically connected to the top electrode of the topmost organic light emitting diode directly over the protrusion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an organic electroluminescent device, and moreparticularly to a tandem organic electroluminescent device.

2. Description of the Related Art

Recently, with the development and wide application of electronicproducts, such as mobile phones, PDA, and notebook computers, there hasbeen increasing demand for flat display elements which consume lesselectric power and occupy less space. Among flat panel displays, organicelectroluminescent devices are self-emitting, and highly luminous, withwider viewing angle, faster response, and a simple fabrication process,making them the industry display of choice.

An organic light-emitting diode (OLED) is a light-emitting diode thatuses an organic layer as the active layer. In recent years, OLEDs havebeen gradually employed in flat panel displays. The trend in organicelectroluminescent display technology is for higher luminescentefficiency and longer lifetime.

Recently, in order to enhance the brightness of the single pixel andattach full color display, a so-called “tandem type organicelectroluminescent device” has been provided. The tandem type organicelectroluminescent device comprises a plurality of organiclight-emitting diodes which are vertically stacked and cascadedtogether.

FIG. 1 is a cross section of a conventional tandem type organicelectroluminescent device 10. The conventional tandem type organicelectroluminescent device 10 comprises a first organic light-emittingdiode 20, and a second organic light-emitting diode vertically stackedabove the first organic light-emitting diode 20.

Specifically, the first organic light-emitting diode 20 comprises afirst bottom electrode 21, a first organic electroluminescent layer 22,and a first top electrode 23, and the second organic light-emittingdiode 30 comprises a second bottom electrode 31, a second organicelectroluminescent layer 32, and a second top electrode 33.Particularly, in the conventional tandem type organic electroluminescentdevice 10, the first top electrode 23 is electrically connected anddirectly contacts the second bottom electrode 31.

Since the resistance of electrode (conductive layer) is lower than thatof organic electroluminescent layer, carriers are apt to migrate withinthe electrodes rather than passing through the boundary energy barriersof the interfaces between electrodes and organic electroluminescentlayers, resulting in crosstalk 40 between adjacent pixels, as shown inFIG. 2.

In order to solve the aforementioned problems, a conventional tandemtype organic electroluminescent device with very thinner intermediateelectrode has been provided, wherein the thinner intermediate electrodeprovides a sheet resistance about ten to dozens of thousands ohms.However, the electrode is thin enough that reproductively thereof ispoor. Another conventional tandem type organic electroluminescent devicewith patterned intermediate electrode has also been provided to preventthe pixels from crosstalk. The electrode patterning process employingshadow mask is not suitable for use in large display fabrication.

Another tandem type organic electroluminescent device e with layerconnecting each OLED device is provided. To enhance efficiency, theconnection layer has both electron and hole transport abilities and highoptical transmittance.

The connection layer can be a doping organic layer such as N-type dopingorganic layer, P-type doping organic layer or combinations thereof.However, the performance of tandem type organic electroluminescentdevice depends on the stability of the connection layer. Since thetandem type organic electroluminescent device utilizes high operatingvoltage, the carrier transport ability of the connection layer isweakened due to carrier diffusion effect.

US Patent NO. 20030189401 discloses a tandem type organicelectroluminescent device 100 having a charge generating layer (CGL) asa connection layer, as shown in FIG. 3.

The tandem type organic electroluminescent device 100 comprises asubstrate 110, a bottom electrode 120, a first organicelectroluminescent layer 130, a charge generating layer 140, a secondorganic electroluminescent layer 150, and a top electrode 160.Specifically, the charge generating layer 140 serves as a floatingelectrode rather than being electrically connected to other electrodes.The tandem type organic electroluminescent device 100 exhibits improvedcurrent efficiency and lifetime.

Suitable material of the charge generating layer, however, is currentlyunavailable and usually toxic (such as V₂O₅). Therefore, it is difficultto achieve mass production. Moreover, the charge generating layercomprising V₂O₅ has high resistance, resulting in a high driving voltageof the tandem type organic electroluminescent device.

Therefore, it is necessary to develop a tandem organicelectroluminescent device to solve the aforementioned problems.

BRIEF SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide a novel tandemorganic electroluminescent device, solving the aforementioned problems,without increased process complexity.

An exemplary embodiment of such a tandem organic electroluminescentdevice comprises a substrate having a pixel thin film transistor. A ribwith chambered corners is formed on the substrate, surrounding a displayregion. A protrusion is formed in the display region. A plurality oforganic light emitting diodes is, stacked vertically in the displayregion, covering the protrusion, wherein each organic light emittingdiode comprises a top electrode, an organic electroluminescent layer,and a bottom electrode. The bottom electrode of the bottommost organiclight emitting diode is electrically connected to the pixel thin filmtransistor. A common electrode is electrically connected to the topelectrode of the topmost organic light emitting diode directly over theprotrusion.

Particularly, the protrusion has a height not less than that of the ribwith chambered corners, and the rib with chambered corners has a heightnot less than that of the stacked vertically organic light emittingdiodes.

According to another exemplary embodiment of the invention, a tandemorganic electroluminescent device comprises a substrate having a pixelthin film transistor. A rib with chambered corners is formed on thesubstrate, surrounding a display region. A plurality of organic lightemitting diodes is stacked vertically in the display region, whereineach organic light emitting diode comprises a top electrode, an organicelectroluminescent layer, and a bottom electrode. The bottom electrodeof the bottommost organic light emitting diode is electrically connectedto the pixel thin film transistor. A common electrode is formed over therib, wherein the top electrodes of two adjacent topmost organic lightemitting diodes are electrically connected by the common electrode.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 shows a cross section of a conventional tandem type organicelectroluminescent device.

FIG. 2 is a circuit diagram of the conventional tandem organicelectroluminescent device.

FIG. 3 is a cross section of a conventional tandem type organicelectroluminescent device with charge generating layer.

FIG. 4 is a partial schematic diagram of a tandem organicelectroluminescent device according to an embodiment of the invention.

FIG. 5 is a cross section of FIG. 4 along line A-A′.

FIG. 6 is a partial schematic diagram of a tandem organicelectroluminescent device according to another embodiment of theinvention.

FIG. 7 is a cross section of FIG. 6 along line B-B′.

FIG. 8 is a cross section of a sealing structure according to a tandemorganic electroluminescent device.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 is a partial schematic diagram of a tandem organicelectroluminescent device according to an embodiment of the invention.The tandem organic electroluminescent device comprises a plurality ofpixel areas 200 arranged in a matrix.

Each pixel area 200 comprises a thin film transistor 211 electricallyconnected to a data line 212 extending along a Y axis, a scan line 213extending along an X axis, a capacitor 214, a pixel thin film transistor215, and a rib 217 with chambered corners. The rib 217 with chamberedcorners is formed on the substrate, surrounding a display region 218.Particularly, the pixel area 200 further comprises a protrusion 219formed within the display region 218. Further, the pixel area 200further comprising a plurality of organic light emitting diodes stackedvertically in the display region 218, covering the protrusion 219. Eachorganic light emitting diode comprises a bottom electrode, an organicelectroluminescent layer, and a top electrode (from bottom to top).Particularly, the bottom electrode of the bottommost organic lightemitting diode is electrically connected to the pixel thin filmtransistor 215. Further, the top electrode of the topmost organic lightemitting diode is electrically connected to a common electrode (notshown).

FIG. 5 is cross section of FIG. 4 along line A-A′. Herein, the pluralityof organic light emitting diodes can comprise two organic light emittingdiodes stacked vertically. In some embodiments of the invention, theplurality of organic light emitting diodes can comprises at least twoorganic light emitting diodes) stacked vertically, such as three or fourorganic light emitting diodes stacked vertically.

Referring to FIG. 5, the tandem organic electroluminescent devicecomprises a first organic light emitting diode 230 and a second organiclight emitting diode 240. The first and second organic light emittingdiodes 230 and 240 are formed on the substrate 210 after forming the rib217. The rib has a height of 10 nm and 10,000 nm and can be transparentmaterials such as polyimide, patterned by photolithography.

The method for forming the first and second organic light emitting diode230 and 240 and the protrusion 219 comprises a first electrode 231formed on the substrate 220 and electrically connected to the pixel thinfilm transistor. After patterning of the first electrode 231, theprotrusion 219 is formed on the substrate 210 within the display region218, wherein the protrusion 219 has a height h1 not less than the heighth2 of the rib 217 with chambered corners. The height h1 of protrusion219 is preferably between 10 nm and 10,000 nm. It should be noted thatthe protrusion has tapered sidewalls, resulting in subsequent layerconformally disposed thereon. Next, a first organic electroluminescentlayer 232 and a first top electrode 233 are sequentially formed on thefirst electrode 231, wherein the first bottom electrode 231, the firstorganic electroluminescent layer 232, and the first top electrode 233comprise the first organic light emitting diode 230. The second bottomelectrode 241, the second organic electroluminescent layer 242, and thesecond top electrode 243 are sequentially formed on the first topelectrode 233, wherein the second bottom electrode 241, the secondorganic electroluminescent layer 242, and the second top electrode 243comprise the second organic light emitting diode 240. The first andsecond organic electroluminescent layers 232 and 242 comprise polymerelectroluminescent material or small molecule electroluminescentmaterial.

After the organic light emitting diodes 230 and 240 are stackedvertically, a sealing case 270 having a common electrode 260 formedbeforehand is bonded to the substrate 210, resulting in the second topelectrode 243 electrically connecting to the common electrode 260. Asshown in FIG. 8, the sealing case 270 is mounted on the substrate bymeans of an electrically conductive adhesive 271, wherein the commonelectrode 260 is electrically connected to a contract pad 272 viaelectrically conductive adhesive 271.

FIG. 6 is a partial schematic diagram of a tandem organicelectroluminescent device according to another embodiment of theinvention. The tandem organic electroluminescent device comprises aplurality of pixel areas 200 arranged in a matrix. As shown in FIG. 7illustrating a cross section of FIG. 6 along line B-B′. Herein, the rib217 is separated form the first top electrode 233 and the second bottomelectrode 241 within the adjacent pixel areas 200, thereby preventingcrosstalk. Moreover, in this embodiment, a thicker metal layer serves asthe second top electrode 243, and the adjacent pixels areas areelectrically connected via the second top electrode 243, thereby coupledto Vss. Therefore, it is not necessary to additionally form a commonelectrode. As a main feature and a key aspect, the height h3 of thesecond top electrode 243 directly over the pixel area 218 is larger thanthe height h2 of the rib 217, referring to FIG. 7. The conductive layer266 and the second top electrode 243 can be of the same material anddeposited by the same process. The method for forming the conductivelayer 266 comprises forming a metal layer and then planarizing for themetal layer.

According to the invention, tandem organic electroluminescent deviceswith novel structures are provided to solve the aforementioned problemsposed in convention tandem type organic electroluminescent device withcharge generating layer (CGL). Since the rib has chambered corners (asshown in FIG. 7), the first top electrode 233 and the second bottomelectrode 241 within the adjacent pixel areas 200 are seperated,preventing the adjacent pixels areas 200 from crosstalk.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1. A tandem organic electroluminescent device, comprising: a substratehaving a pixel thin film transistor; a rib with chambered corners on thesubstrate, surrounding a display region; a protrusion in the displayregion; a plurality of organic light emitting diodes stacked verticallyin the display region, covering the protrusion, wherein each organiclight emitting diode comprises a top electrode, an organicelectroluminescent layer, and a bottom electrode, and the bottomelectrode of the bottommost organic light emitting diode is electricallyconnected to the pixel thin film transistor; and a common electrodeelectrically connected to the top electrode of the topmost organic lightemitting diode directly over the protrusion.
 2. The device as claimed inclaim 1, wherein the protrusion has a height not less than that of therib with chambered corners.
 3. The device as claimed in claim 1, whereinthe rib with chambered corners has a height not less than that of thevertically stacked organic light emitting diodes.
 4. The device asclaimed in claim 1, wherein the organic electroluminescent layercomprises polymer electroluminescent material.
 5. The device as claimedin claim 1, wherein the organic electroluminescent layer comprises smallmolecule electroluminescent material.
 6. The device as claimed in claim1, wherein the organic electroluminescent layer consists of a singleorganic material layer.
 7. The device as claimed in claim 1, wherein theorganic electroluminescent layer consists of a plurality of organicmaterial layers.
 8. The device as claimed in claim 1, wherein theprotrusion has a height of 10 nm˜10,000 nm.
 9. The device as claimed inclaim 1, wherein the rib has a height of 10 nm˜10,000 nm.
 10. The deviceas claimed in claim 1, wherein the protrusion has a tapered sidewall.11. The device as claimed in claim 1, wherein the plurality of organiclight emitting diodes comprises at least two organic light emittingdiodes stacked vertically.